Ghrelin (also referred as “acylated ghrelin” or abbreviated as “AG”) is a 28 amino acid peptide, purified and identified from rat stomach and characterized by the presence of an n-octanoyl modification on the Ser3 residue (Ref. 1). Acylation of ghrelin is catalyzed by the enzyme ghrelin O-acyl transferase (GOAT). The expression of GOAT is mostly in the stomach and intestine. Ghrelin is the endogenous ligand of the growth hormone (GH) secretagogue receptor (GHSR-1a) (Refs. 2, 3). Ghrelin is now mostly recognized as a potent orexigenic factor stimulating food intake and modulating energy expenditure (Refs. 4, 5 and 6). At the peripheral level, Ghrelin exerts probably its major physiological action regulating glucose and lipid metabolism (Ref. 7). In fact, ghrelin has a diabetogenic action (Ref. 8) and suppresses glucose-stimulated insulin secretion and deteriorates glucose tolerance (Ref. 9).
As such, elevated plasma ghrelin is of relevance in certain disorders of the metabolism and growth such as in diabetes and obesity. Elevated plasma ghrelin levels have also been demonstrated amongst adults and children with Prader-Willi Syndrome (PWS) (Ref. 10 and 11). PWS is a genetic obesity syndrome associated in most patients with GH deficiency. Children with PWS present a rapid weight gain along with a voracious appetite. Studies on the involvement of ghrelin in PWS have provided a significant rationale that the hyperphagia observed in PWS is positively correlated with elevated ghrelin levels, consistent with the known orexigenic effect of ghrelin (Ref. 12).
Unacylated ghrelin (also referred as “des-acyl ghrelin” or abbreviated as “UAG”), is the non-acylated form of ghrelin. Its concentration in plasma and tissue is higher compared to ghrelin. UAG has long been considered as a product with no physiological role as it fails to bind the only known ghrelin receptor GHSR-1a at physiological concentrations and has no physiological effect on GH secretion (Ref. 15). However, UAG is a biologically active peptide, particularly at the metabolic level and its administration has been shown to induce a negative energy balance by decreasing food intake and delaying gastric emptying (Ref. 16). Over-expression of UAG in mice results in a decrease in fat accumulation with an increase in insulin sensitivity and glucose tolerance (Refs. 16 and 17).
UAG has been shown to prevent the hyperglycemic effects of ghrelin, when administered concomitantly, in healthy volunteers, see in particular U.S. Pat. No. 7,825,090, herein incorporated in its entirety by reference. This initial observation was followed by several reports on the anti-diabetogenic potential of UAG (Refs. 18, 19, 30, 31 and 32).
In vitro, in vivo and clinical evidence indicate that UAG prevents the diabetogenic effects of ghrelin in healthy volunteers and in GH-deficient patients (Refs. 18 and 19). It inhibits both basal and ghrelin-induced glucose secretion by human hepatocytes (Ref. 31). In rats, UAG enhances portal insulin response to glucose (Ref. 32) and reduces fat deposition and triglycerides levels, as observed in transgenic mice overexpressing UAG (Ref. 16). In vitro, UAG stimulates insulin secretion from insulinoma cells (Ref. 32) and promotes proliferation and inhibits apoptosis of beta cells (Ref. 33).
The anti-diabetogenic effects and ghrelin-antagonizing effects of UAG, fragments and analogs thereof have been reported in U.S. Pat. No. 7,485,620; U.S. Pat. No. 8,222,217; U.S. Pat. No. 8,318,664 and in WO 2008/145749, which are all in their entirety incorporated herein by reference.
Recent experiments on circulating angiogenic cells (CAC) indicates that UAG beneficially impacts the vascular remodeling process which is known to be impaired in type 2 diabetes patients. The effects of UAG on CAC have been reported in U.S. Patent Application Serial Number 2010/0016226 and in WO 2009/150214, herein incorporated in their entirety by reference.
Obese mice and humans have been reported to present lower UAG levels than normal weight subjects, indicating that obesity might be correlated with a relative UAG deficiency (Refs. 34, 35 and 21). It has been observed that insulin-resistant obese subjects have an elevated AG/UAG ratio when compared to insulin-sensitive obese subjects (Refs. 20 and 22).
Treatments that target ghrelin and the GHS-R (i.e., ghrelin antagonists) have been suggested as attractive pharmacologic avenues to fight against obesity and other conditions, disorders and diseases associated with ghrelin. Several GHS-R ligands and anti-obesity vaccines have been proposed (Ref. 24). Other pharmacological approaches inducing antibodies against ghrelin, ghrelin enantiomers and inhibition of ghrelin acyl-transferase (GOAT) (Ref. 25) have been investigated; however, due to lack of efficacy, non-selectivity and lack of sustained weight loss, these pharmacological approaches have not yet reached the market (Ref. 26).
Therefore, there exists a need in the art for an efficient and more direct way of modulating circulating ghrelin levels and/or circulating ghrelin/unacylated ghrelin ratio in subjects wherein such modulation is beneficial to the subject and for more efficient ways of identifying those subjects that can benefit from modulation of ghrelin levels and ghrelin/unacylated ghrelin ratio.